Unsaturated polyester compositions which are condensation polymers based on glycols and dibasic acids are used in a multitude of applications. These unsaturated polyester resins are essentially Newtonian in nature, that is, their viscosity is not shear dependent. In many of the applications of such resins, the resin must not drain from vertical surfaces before cure and must possess other properties of acceptable thixotropic character, which means that the resin should exhibit high apparent viscosity at low shear and much lower viscosity at high shear, and must also not drain from a vertical surface before cure.
Thixotropic polyester resins and gel coats are widely used in the marine industry as fiberglass reinforced plastics, such as in boat building, corrosion-resistant products and electrical and transportation applications. See "Modern Plastics Encyclopedia", 1977-1978, p. 65-66; and 1978-1979, p. 56-59. These are special fields in which thixotropic resins must be used which meet the requirements set forth above for thixotropic character. See Connolly, "Modern Plastics Encyclopedia", 1975-1976, p. 61-62/ Heretofore, fumed silicas have been widely used for this purpose. See the paper entitled "Thixotropic Behavior of Fumed Silicas in Gel Coats", by Jurgen Hoell, 32nd Annual Technical Conference, 1977, Reinforced Plastics/Composite Institute, The Society of Plastics Industry, Inc.; Section 17-D, p. 1-8. It is well-known, however, that the rheological properties of fumed silicas are highly dependent on the extent of shear and the degree of dispersion. Since the functional performance of the fumed silicas is shear-dependent, it is possible to overdisperse or underdisperse the fumed silicas and consequently produce thixotropic laminating resins with batch to batch variations in rheological properties. Other problems which have been encountered from time to time by the polyester industry with fumed silicas involve their settling properties, cobalt promoter adsorption, lack of product uniformity, moisture sensitivity and the lack of long-term viscosity stability.
The prior art is also aware that materials other than fumed silica have been incorporated into unsaturated polyester compositions, although other inorganic materials have been employed as thixotropic agents. For example, in U.S. Pat. No. 4,081,496, organophilic clay gellant compositions have been incorporated into polyester resins to provide a thixotropic product. In addition, in U.S. Pat. No. 4,008,294, ultrafine polypropylene fibers in epoxy resin compositions have been used to cause the resin composition to have thixotropic characteristics.
In U.S. Pat. No. 3,402,151, zeolite molecular sieves were incorporated into polysulfide compositions to increase storage stability of the compositions. Similarly, in U.S. Pat. No. 3,419,533, it is disclosed that fillers and plasticizers such as carbon black, precipitated silicic acid, silicic acid sols, clay and the like may be incorporated into polyurethane plastics as fillers and plasticizers, but without suggestion as to any thixotropic characteristics. On the other hand, in U.S. Pat. No. 3,607,822, colloidal silica was incorporated into polyurethane compositions to provide a thixotropic polyurethane composition.
There is accordingly a need remaining in the art for a product which can provide advantageous thixotropic properties to unsaturated polyester resins. The present invention provides a composition which meets this need.